A light-emitting diode (referred to hereinafter as LED) represents one of the most popular light-emitting devices today. In recent years, the luminous efficacy of LEDs, defined in lumens per Watt, has increased significantly from 20 lumens per Watt (approximately the luminous efficacy of an incandescent light bulb) to over 200 lumens per Watt, which greatly exceeds the luminous efficacy of a fluorescent light at 60 lumens per Watt. In other words, for a fixed amount of light output, LEDs consume only one third the power compared to fluorescent lights, and one tenth compared to incandescent light bulbs. Accordingly, it is not surprising today that lighting fixtures with LEDs have recently been replacing incandescent light bulbs and fluorescent light tubes. A new term “Solid-State Lighting” has been created. The term “Solid-State Lighting” refers to the type of lighting that uses semiconductor light-emitting diodes, such as an LED rather than traditional light sources.
In the field of solid-state lighting, most of the light sources are white light. For LEDs, white light may be obtained by a method referred to as “RGB white”. In this method, colored light emitted by LEDs, usually in primary colors such as red, green and blue light, is mixed to obtain white light. Another method is to utilize a wavelength-converting material to convert a narrow band light into a broad-spectrum white light. For example, white light can be obtained by converting a narrow band blue or ultra-violet light emitted by an LED die by passing the narrow band light through yellow phosphor. This method is also known as “phosphor converted white”. Generally, phosphor converted white method is more popular than the RGB white method, because the RGB white method requires at least three types of LEDs and careful consideration on light mixing.
Most light sources used in solid-state lighting may be further categorized by color temperature. The color temperature of a light source indicates the relative color appearance of the particular light source on a scale from “warmer” (more yellow/amber) to “cooler” (more blue) light. Color temperatures are generally given in Kelvin or K. Color temperatures over 5,000 K are called cool colors (bluish white), while lower color temperatures (2,700-3,000 K) are called warm colors (yellowish white through red). For phosphor converted white method, the color point of the light is determined by the type of phosphor or the composition of the phosphor if more than one type of phosphor is utilized. In most white LED devices, the phosphor may be mixed with an encapsulant. The phosphor is often in direct contact with the light source die. However, the light source is known to be a heat source of the light-emitting device. This may not be desirable because the heat may affect the phosphor material. In addition, the color point of a phosphor converted white LED may be difficult to control and often the process variation may be huge. Generally, one solution is to bin the color temperature and the brightness of the LEDs so that LEDs with similar brightness can be obtained and fixed into lighting fixture. The binning process resulted in production yield loss as the process variation for phosphor may be huge.
From lighting fixture manufacturer's perspective, the binning is not desirable. In order to fulfill the market needs of a wide range of color temperature ranging from warm white lighting fixtures to cool white lighting fixtures, lighting fixture manufacturers may have to manage significant inventories as the LEDs are categorized into many small color and brightness bins for each color temperature.